The current analog video interface used in the personal computer (PC) industry is commonly referred to as the VGA interface and has served for over 15 years in the PC industry. This interface continues to be the de facto standard video connection and is still used with the vast majority of displays and graphics hardware sold today.
The current interface or VGA standard is based on a 15-pin high density D subminiature connector with a standardized pin-out as shown in FIG. 6. In addition, the following basic specifications are established for this system:
A. There are three video signals, providing luminance information for each of three primary color channels (Red, Green, and Blue). These are positive white signals where increasing the positive signal voltage with respect to the reference increases the luminance of that channel on the display. The signals have an amplitude of approximately 0.7V p-p, with an impedance of 75 ohms, and the signals are assumed to be AC coupled in order to block certain levels of DC voltages. The reference level for such signals are established by requiring that all three of these channels be at a defined “blanking” level during the time around the horizontal sync pulse, at which time the display will “clamp” or set an internal reference to this level. Each video signal is provided with a dedicated return line or ground.
B. In the VGA standard, timing information is not directly provided by the video signals themselves. Instead, horizontal line and vertical frame or field synchronization signals (syncs) are provided in the form of separate TTL signal lines, each on their own pin but sharing a common return.
C. Display identification and control is provided through a general purpose communications channel, established by the VESA Display Data Channel standard. This occupies pins 9, 12, and 15.
Despite its widespread use and relatively long history, this analog interface suffers from several shortcomings. One problem is its suitability for use with fixed-format displays, such as liquid crystal displays (LCDs). Another deficiency with analog interfaces is that there is generally no means of implementing an automatic gain control. There is a certain amount of signal attenuation from cable losses, circuit variations, and similar losses. This attenuation can be corrected by increasing the gain on the monitor side of the interface. Of course, users of an analog video monitor can manually adjust the gain control when such controls are provided. Even if the gain is manually adjustable, the user does not have a reference point upon which to base their adjustment. Without a reference, any modifications to the gain are merely arbitrary and the user has no idea what gain setting allows the picture to be viewed at the brightness level originally intended by the system's video adapter or graphics card.
Newer and more capable interfaces have been introduced in an attempt to address the shortcomings of the VGA interface. Two of the more widely recognized standards are the Plug & Display (P&D) standard from the Video Electronics Standards Association (VESA), and the Digital Visual Interface (DVI) standard from the Digital Display Working Group (DDWG). Both the P&D and DVI standards have offered a generally digital interface for use with non-CRT displays, under the belief that such displays are more suited to a digital form of video transmission.
These standards have seen very limited acceptance, primarily due to the lack of compatibility with the earlier VGA standard. Unfortunately, this means that display systems will generally continue to use the VGA interface despite its limitations.